1. Field of the Invention
The present invention relates to estimating distortion levels in a channel of a communications system.
2. Related Art
Conventional communications systems, such as a Data Over Cable Service Interface Specification (DOCSIS) based upstream systems, often receive burst communications. To provide channel fidelity, such as adequate signal to noise ratio (SNR) and signal power to distortion power ratio, many of these DOCSIS based systems (e.g., receivers) provide channel equalization. For short bursts that occur within suboptimal receivers, equalized receiver channels may provide little, if any, improvement over un-equalized channels. Thus, channel fidelity of short bursts, especially in the case of these suboptimal receivers, will be problematic.
During short bursts, the practicality of using equalizer techniques such as adaptive equalization, may be limited because the bursts may be too short to trigger or retain equalization benefits provided within the channel. For example, many users may be contending for available channels within the system, with each channel employing unique user dependent equalization techniques.
The unique equalization technique employed is typically predicated upon receipt and analysis of user data that is at least of minimum duration. A short communications burst, however, may not meet this minimum duration criteria. Therefore, although these short bursts may occur in an equalized channel, they can occur without the benefit of equalization when their duration is shorter than the required minimum. One method in the current art for overcoming this problem is for the subscriber to send periodic training or ranging bursts, which contain largely known symbol patterns or training sequences. The receiver uses these training sequences to estimate the channel response and spectrum of any interference and noise on the channel. The receiver then downloads equalizer coefficients to a pre-equalizer in the subscriber transmitter. This method of pre-equalization benefits both short and long packets sent by the subscriber.
Before the challenge of ensuring adequate channel fidelity can be remedied, it must be quantified. That is, before system designers and engineers can provide adequate channel fidelity for all users, they should especially understand the extent to which short burst communications can become degraded, or distorted. In addition, there may be historical or economic reasons why equalization is not practicable in a given communications system. For example, the installed base of legacy modems may not support pre-equalization.
One traditional technique for understanding channel distortion associated with short bursts, involves estimating a performance metric, such as the signal power to distortion power ratio. More specifically, this traditional technique includes estimating the signal power to distortion power ratio in a communications channel devoid of equalization. Although this traditional technique can be implemented using several different approaches, none of the resulting estimations are particularly reliable.
One other traditional approach to estimating channel fidelity includes designing more complex receivers. More complex receivers, however, are less desirable because of factors such as cost, speed, and power consumption. Also, since many communication systems include thousands of channels. So more complex hardware and complex approaches that require, for example, special data development techniques, are undesirable.
What is needed, therefore, is a system and method for estimating the fidelity, or SNR, of a communications channel used without equalization.